So my 2 year old cat has been acting strange for a few days. He has been more aggressive with my 1 year old cat (usually its just "you got too close and I don't like it" or "let's wrestle and maybe hiss") but every time he even sees him its hissing and growling (they have lived together since September of last year). He's also become more aggressive with me. He's always been independent but I can't seem to walk past him sometimes without receiving a hiss. He's also been biting more. I thought it might be that his FLUTD (feline lower urinary tract disease) flaring so I gave him some of his pain medication but that doesn't seem to be the problem. Now he's acting much more withdrawn and doesn't want affection (he usually comes to see me when he wants cuddles or wants to take a nap on my lap). I saw him eat this morning but I'm not sure if I'm reading too much into it or if there may actually be something wrong. Any advice is appreciated!